Recent advances in micro-electro-mechanical systems (MEMS) technology, wireless communications, and digital electronics have enabled the development of low-cost, low-power, multifunction sensor nodes that are small in size and communicate un-tethered in short distances. These tiny sensor nodes, which consist of sensing, data processing, and communicating components, leverage the idea of sensor networks based on the collaborative effort of a large number of nodes.
A sensor network is composed of a large number of sensor nodes. Sensor networks may consist of many different types of sensors such as seismic, low sampling rate magnetic, thermal, visual, infrared, acoustic, radar, etc. that are able to monitor a wide variety of ambient conditions.
Sensor nodes can be used for continuous sensing, event detection, event ID, location sensing, and local control of actuators. The concept of micro-sensing and wireless connection of these nodes promise many new application areas. Example applications include military, environment, health, home and other commercial areas. It is possible to expand this classification with more categories such as space exploration, chemical processing and disaster relief.
FIG. 1 illustrates one example a sensor network. As shown, a plurality of sensor nodes 10 are distributed in a sensor field 12. When a sensor node 10 senses, for example, an event, the sensor node 10 attempts to transmit a message containing information regarding the event to a sink 16. The sink 16 may be a gateway for a communication network 18 such as the internet, a wireless communication network, a satellite network etc. The message is routed by the communication network 18 to a task manager 20 (e.g., a server or computer system), which performs a task based on the information contained in the message.
A sensing node 10 that transmits a message after sensing, for example, an event is referred to as a source node 14. In attempting to send the message to the sink 16, the source node 14 may not transmit at a high enough power, or be located close enough to the sink 16, for the sink 16 to directly receive the message from the source node 14. Accordingly, the sensor nodes 10 are configured to receive messages from the other sensor nodes 10 located nearby and relay the received message to other nearby sensor nodes 10 such that the messages make forward progress towards the sink 16. The sensor nodes 10 via which the message travels from the source node 14 to the sink 16 are referred to as relay nodes 22. Each sensor node 10 can assume the role of source, relay and/or sink. Each transfer of the message from one sensor node 10 to another sensor node 10 is referred to as a hop, and the amount of time to complete a hop is referred to as the hop latency.
One of the challenges faced in sensor networks is minimizing power consumption. Sensor nodes are often inaccessible, and the lifetime of the sensor nodes, and thus the sensor network, depends on the lifetime of the power resources (e.g., battery life) for the sensor nodes. For adhoc networking (e.g., sensor networks), energy efficiency can be achieved by techniques that minimize transmission and reception processing power at the nodes (e.g., sensor nodes) where processing power has been classified into two classes: idle and receive. When, transmit/receive states of the nodes are uncommon due to infrequent messaging, significant energy savings can be achieved by allowing the nodes to enter a sleep mode of operation where no receiver processing is performed. Research in sleep patterns has typically considered two cases: nodes with asynchronous, independent sleep patterns and synchronous sleep/wake cycles between nodes in a local neighborhood.
In an asynchronous system, when a node wakes up, it has to stay awake for a substantial period of time in order to catch the retransmission of messages that arrived while the node was asleep. The overall duty-cycle of a node (and thus its energy consumption) can only be lowered by about 50% in order to avoid excessive overhead caused by extremely small retransmission time-out values. The need of an extended listening period of a newly awake node may be eliminated by requiring an active node to stay awake and listen continuously until its energy is depleted. While this approach can eliminate the receiver wake-up latency, it does not exploit the potential energy savings of switching off the transmission/reception processing of a node while keeping its sensing/monitoring processing active.
Energy consumption can be reduced by synchronizing all nodes in a neighborhood to wake up in the same time-slot within a multi-slot frame. However, because all message exchanges within a neighborhood can only be conducted once every frame, the average per-hop latency is half of the frame interval. Excessive collisions (and thus delay) is likely to occur when multiple node-pairs within the same neighborhood try to communicate with each other during the same frame.
While the sleep mode can provide significant savings in energy, it can also introduce significant delays and impair the ability of the multiple node network to convey a message in a timely manner. Most existing sleeping approaches do not support flexible trade-offs between energy savings and latency.